Computed tomography using broadband Bessel THz beams and phase contrast.

We present new results demonstrating the capability of performing computed tomography (CT) using broadband Bessel terahertz (THz) beams. Nondiffractive beams such as these exhibit propagation-invariant lines of focus with an extended depth-of-field compared to conventional Gaussian beams. Using this property, we demonstrate a considerable improvement in the 3D reconstruction image of a synthetic sample through the backprojection algorithm. Only when THz Bessel beams are used, a full reconstruction of the object structure is made. Moreover, we use phase-contrast mechanism which improves the spatial resolution and reconstructed images. Our results highlight the potential in using nondiffractive Bessel beams to significantly improve 3D-image reconstruction of THz CT.

Improving depth-of field in broadband THz beams using nondiffractive Bessel beams.

We report new results related to imaging using broadband Bessel-like beams at the terahertz (THz) domain that were generated by use of axicons and pulsed THz radiation emitting at a bandwidth 0.1 to 1 THz. Such Bessel-like beams exhibit an invariant line of focus with an extended length compared to Gaussian-beams Rayleigh range, which enables imaging through the extended length. We demonstrate this imaging property using a resolution target illuminated by broadband-THz beams and show an improvement by a factor of 3.5 in imaging depth while using Bessel-like beams over Gaussian beams. Our results highlight the potential in using broadband THz radiation together with nondiffractive Bessel beams to significantly improve spatial separation over deep view.

Use of polycrystalline Nd:YAG rods to achieve pure radially or azimuthally polarized beams from high-average-power lasers.

We report maintenance of perfect radial-polarization purity in high-power, cw pump chambers using strengthened, polycrystalline Nd:YAG laser rods. Although the cubic symmetry of single-crystal rods caused threefold symmetric birefringence due to shear stresses at the ends of the pumped zone, polycrystalline rods macroscopically behaved as isotropic material and enabled polarization preservation. Elimination of this source of depolarization prevents the major source of bifocusing aberrations in a chain of amplifiers.

Thermally induced birefringence in nonsymmetrically pumped laser rods and its implications for attainment of good beam quality in high-power, radially polarized lasers.

The ability to generate and then amplify radially polarized light opens up the possibility of achieving very high-power, near diffraction-limited beams from rod-based solid-state lasers. Residual bifocusing rapidly degrades beam quality. Residual bifocusing results from nonradially symmetric pump distributions. We analyze how a nonradially symmetric pump distribution induces a nonradially symmetric stress map. This manifests itself as nonradially symmetric birefringence, and as depolarization to radially/azimuthally polarized beams (or as deterioration in birefringence compensated linearly polarized lasers). Here we analytically describe the birefringence terms of a nonradially symmetric strain map. The model results are supported by radial-depolarization measurements in our 2 kW Nd:YAG pump chambers. For the current level of depolarization, beam quality degradation per rod is DeltaM(2)=4 because of bifocusing alone. The degradation per rod can be reduced substantially by improving pump uniformity.

Radial and azimuthal beam parameters.

Global invariant parameters are introduced to characterize the radial and azimuthal content of totally polarized beams. Such parameters are written in terms of the second moments of the optical beam and are invariant in propagation through symmetric first-order optical systems described by the ABCD matrix. Since it was proven in the past that the usual definition for radial polarization is not invariant, such invariance is novel in characterizing the radial and azimuthal polarizations content of optical beams. The possibility of obtaining a pure mode from a given beam using the proposed parameters is discussed.

2 kW, M2 < 10 radially polarized beams from aberration-compensated rod-based Nd:YAG lasers.

Radially polarized light in a 2.1 kW, good quality beam was obtained from a Nd:YAG rod-based master oscillator power amplifier. Several techniques were utilized: a pure radially polarized oscillator, efficient pump chambers, external compensation of lower-order aberrations, and higher-order aberration compensation by pairing of pump chambers.

Birefringence-induced bifocusing for selection of radially or azimuthally polarized laser modes.

We develop a round-trip matrix diagonalization method for quantitative description of selection of radially or azimuthally polarized beams by birefringence-induced bifocusing in a simple laser resonator. We employ different focusing between radially and tangentially polarized light in thermally stressed laser rods to obtain low-loss stable oscillation in a radially polarized Laguerre-Gaussian, LG(0,1)*, mode. We derive a free-space propagator for the radially and azimuthally polarized LG(0,1)* modes and explain basic principles of mode selection by use of a round-trip matrix diagonalization method. Within this method we calculate round-trip diffraction losses and intensity distributions for the lowest-loss transverse modes. We show that, for the considered laser configuration, the round-trip loss obtained for the radially polarized LG(0,1)* mode is significantly smaller than that of the azimuthally polarized mode. Our experimental results, obtained with a diode side-pumped Nd:YAG rod in a flat-convex resonator, confirm the theoretical predictions. We achieved a pure radially polarized LG(0,1)* beam with M(2)=2.5 and tens of watts of output power.

Correction of spherical and azimuthal aberrations in radially polarized beams from strongly pumped laser rods.

Spherical aberration of heavily pumped Nd:YAG rods was corrected by use of spherical relay optics selected to add conjugate amounts of aberration. Wavefront measurements showed elimination of spherical aberration. Correction of spherical aberration allowed 250 W of power to be generated in a radially polarized, birefringence-free oscillator (60% more power than without correction). Scale-up of wavefront maintenance was demonstrated in a two-rod amplifier module (6.3 kW electrical pump power). Radial polarization and spherical aberration correction together eliminated the main aberrations in uniformly pumped rod-based lasers. Rotating adjacent pump chambers substantially reduced the multifold aberrations induced by nonuniformity of the azimuthal pumps. DeltaM2 in the radially polarized beam was 0.3 and 1.4 with and without aberration correction, respectively, in each two-pump chamber module. Analysis predicts further improvements when higher-order aberration correction is applied.

Production of radially or azimuthally polarized beams in solid-state lasers and the elimination of thermally induced birefringence effects.

Production and amplification of radially and azimuthally (tangentially) polarized laser beams are demonstrated. Based on the different focusing between radially and tangentially polarized light in thermally stressed isotropic laser rods, Nd:YAG laser oscillators were developed to produce low-loss stable oscillation in a single polarization. Pure radially polarized light at 70 W with M2 = 2 and on-axis impure radially polarized light at 150 W with M2 = 2.5 were achieved. The radially polarized beams were then amplified while good beam quality and polarization purity were retained. Complete elimination of thermal-birefringence-induced aberrations was demonstrated. This should allow much better beam quality from rod-based high-power lasers.

Comparison of adaptive optics and phase-conjugate mirrors for correction of aberrations in double-pass amplifiers.

Correction of birefringence-induced effects (depolarization and bipolar focusing) were achieved in double-pass amplifiers by use of a Faraday rotator between the laser rod and the retroreflecting optic. A necessary condition was ray retrace. Retrace was limited by imperfect conjugate-beam fidelity and by nonreciprocal refractive indices. We compared various retroreflectors: stimulated-Brillouin-scatter phase-conjugate mirrors (PCMs), PCMs with rod-to-PCM relay imaging (IPCM), IPCMs with astigmatism-correcting adaptive optics, and all-adaptive-optic imaging variable-radius mirrors. Results with flash-lamp-pumped, Nd:Cr:GSGG double-pass amplifiers showed the superiority of adaptive optics over nonlinear optic retroreflectors in terms of maximum average power, improved beam quality, and broader oscillator pulse duration/bandwidth operating range. Hybrid PCM-adaptive optics retroreflectors yielded intermediate power/beam-quality results.